Lecture 1: Pressures of the respiratory system – Part 1 of 2

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Aman Thind
Critical care medicine fellow at the Cleveland Clinic. Interests: Cardiopulmonary physiology, shock, POCUS, mechanical ventilation, and ARDS. Music genres: Blues, Rock, and Heavy metal

The Pre-brief

This is the first lecture of the series on respiratory mechanics presented to the Critical Care Medicine fellows at Cleveland Clinic. This lecture is split in two parts and Part B will be uploaded next week.

Post-brief (Notes)

4 pressures at specific locations:

(1.) Airway pressure (Paw) or Pressure at the airway opening (Pao):

– This is the pressure measured by the ventilator in a mechanically ventilated patient

– In a spontaneously breathing patient, Paw/Pao = zero (mouth opening is exposed to atmospheric pressure)

(2.) Alveolar pressure (Palv):

– Pressure within an alveolus.

– Practically not feasible to have a catheter small enough to sit inside an alveolus to directly measure Palv.

– However, there is no pressure differential between airway opening and alveolus. Hence, Pao becomes a reflection of Palv.

– E.g. plateau pressure (end-inspiratory occlusion) and total PEEP (end-expiratory occlusion) are reflectors of Palv at end-inspiration and end-expiration respectively.

(3.) Pleural pressure (Ppl):

– Pressure within the pleural space.

– Can be estimated by using esophageal balloon.

(4.) Body surface pressure (Pbs):

– Pressure outside the body at the body surface.

– Typically equals atmospheric pressure and hence zero. 

4 pressures across specific locations:

(1.) Transairway pressure:

– Pao minus Palv.

– Represents ‘resistive pressure’ in the presence of airflow.

– In the absence of airflow, transairway pressure = zero (Hence, Pao = Palv); see above.

(2.) Transalveolar pressure:

– Palv minus Ppl.

– It is the transmural pressure of the alveolus/lung parenchyma.

– Alveolar inflation is proportional to transalveolar pressure.

– Transalveolar pressure (not Palv) is the true measure of alveolar stress.

(3.) Transchestwall pressure:

– In the absence of muscle activity (Pmus): transchest wall pressure = Ppl – Pbs

– It is the transmural pressure of the chest wall.

(4.) Transpulmonary pressure:

– Transpulmonary pressure = Pao minus Ppl = Transairway pressure minus transalveolar pressure

– In the absence of airflow, transairway pressure becomes zero. Hence, in this state, transpulmonary pressure reflects transalveolar pressure.

– Clinical utility:
   (a) End-inspiratory transpulmonary pressure = Plateau pressure minus end-inspiratory Ppl
        This reflects end-inspiratory transalveolar pressure, and hence, end-inspiratory alveolar stress.
   (b) End-expiratory transpulmonary pressure = total PEEP minus end-expiratory Ppl
        This reflects end-expiratory transalveolar pressure, and hence, end-expiratory alveolar stress.

– In simple words, plateau pressure and PEEP and not true indicators of alveolar stress. Both have to be substracted by pleural pressure (the extramural pressure of the alveolus) to get a better idea of alveolar stress.

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